Optical isolators employing wavelength tuning

Abstract

An improvement in a method of rotating the plane of polarization of polarized light in a Faraday rotator, having an optical element which includes a ferromagnetic material, which comprises varying the strength of the magnetic field generated along the optical axis of the optical element in response to changes in the wavelength of the polarized light. The magnet employed in the Faraday rotator may be at least one permanent magnet or an electromagnet. Preferred optical elements are made of disc having a gadolinium-gallium-garnet (GGG) or large lattice constant (LLC) substrate, and the substrate is coated with an oxygen--and iron--containing film.

What is claimed is: 
     
       1. In a method of rotating the plane of polarization of polarized light in a Faraday rotator, said Faraday rotator including (i) an optical element having an optical axis, said optical element being formed from a composition including at least one ferromagnetic material, and said optical element being formed from at least one substrate in the form of a disc, and each of said at least one disc(s) is coated with an iron-and oxygen-containing film, and (ii) at least one permanent magnet for generating a magnetic field along the optical axis of the optical element, the improvement comprising: varying the strength of magnetic fiels generated along the optical axis of the optical element by moving said at least one permanent magnet with respect to said optical element such that the strength of the magnetic field in said at least one ferromagnetic material is maintained at a level below the saturation point for said at least one ferromagnetic material in order to respond to changes in the wavelength of said polarized light.   
     
     
       2. The method of claim 1 wherein said optical element formed from a composition comprising at least one ferromagnetic material is formed from at least one substrate formed from a material selected from the class consisting of Gd 3  Ga 5  O 2  and Gd 2 .68 Ca O .32 Ga 4 .02 Mg 0 .33 Zr 0 .65 O 12 , and each of said at said at least one substrate(s) is coated with an oxyen-, and iron - containing film. 
     
     
       3. The method of claim 2 wherein said film is formed from (HoTbBi) 3  Fe 5  O 12 . 
     
     
       4. The method of claim 2 wherein said film is formed from (YbTbBi) 3  Fe 5  O 12 . 
     
     
       5. The method of claim 2 wherein said film is formed from Y 3  Fe 5  O 12 . 
     
     
       6. The method of claim 2 wherein said film is a lutetium-iron-garnet film. 
     
     
       7. The method of claim 2 wherein said film is formed from a composition having the formular:   Bi.sub.x RE.sub.3-x Fe.sub.5-y Ga.sub.y O.sub.12,     wherein RE is a rare earth element, x is between 0.4 and 0.6, and y is between 0.8 and 1.2.   
     
     
       8. The method of claim 2 wherein said film is formed from (TbBi) 3  (FeGa) 5  O 12 . 
     
     
       9. The method of claim 2 wherein said film is formed from (GdBi) 3  (FeA1Ga) 5  O 12 . 
     
     
       10. The method of claim 2 wherein said film is formed from (BiLuGd) 3  Fe 5  O 12 . 
     
     
       11. The method of claim 2 wherein said film further contains bismuth. 
     
     
       12. The method of claim 11 wherein said at least one substrate is formed from Gd 3  Ga 5  O 2  and said film is of the formula:   (Tm.sub.2.14 Bi.sub.0.08 Pb.sub.0.07)(Fe.sub.3.13 Ga.sub.1.86)O.sub.12.     
     
     
       13. The method of claim 11 wherein said at least one substrate is formed from Gd 2 .68 Ca 0 .32 Ga 4 .02 Mg 0 .33 Zr 0 .65 O 12  and said film is of the formula:   (Bi.sub.1.09 Tm.sub.0.07 Gd.sub.0.95 Y.sub.0.95)(Fe.sub.3.91 Ga.sub..0.76 Y.sub.0.30 Tm.sub.0.02)O.sub.12.     
     
     
       14. The method of claim 1 wherein said at least one permanent magnet generates a magnetic field along a cylindrical axis, and the strength of said magnetic field generated along the optical axis of the optical element is varied in response to changes in the wavelength of said polarized light by moving said at least one permanent magnet with respect to said optical element such that a portion of said cylindrical axis along which is generated a magnetic field having a magnetic field strength sufficient to achieve a desired rotation of the plane of polarization of polarized light for the particular wavelength of light passing through said optical element, becomes coincident with the optical axis of said optical element.